(a) Field of the Invention
The present invention relates to a cathode ray tube (CRT) and, more particularly, to a flat-panel CRT which can minimize raster distortion of electron beams while maintaining structural strength of a shadow mask.
(b) Description of the Related Art
Generally, a faceplate panel for CRTs is shaped like a convex lens. Both sides of the faceplate panel, an inner phosphor-coated side and an outer viewing screen side, have a curved shape. This is because the convex-shaped panel has advantages in various aspects such as convenience of formation, stability in strength, and adaptability for shadow mask application.
However, to the eyes of the viewer, it is desirable that the screen image should be displayed as substantially flat. For this reason, several attempts have been made to form both sides of the faceplate panel with a flat shape while maintaining the normal display characteristics of the CRT. It is found that when a flat panel is used for the display screen problems occur in the convergence characteristics of electron beams and in the strength of a shadow mask. For example, when the surface of the phosphor-coated side is flat-shaped, it becomes difficult to deflect three electron beams of red R, green G and blue B correctly on a suitable convergence point. Furthermore, because the shadow mask facing the inner side of the panel should be correspondingly flat-shaped, the desired strength of the shadow mask cannot be achieved through the common shadow mask forming technique.
In addition, there is a problem with the flat-panel CRT from the standpoint of the viewer. When the viewer watches a monitor with the flat-shaped faceplate panel, they feel that the screen image is sunken at its center portion while protruded at its peripheral portion.
Therefore, it is preferable in the shadow mask-formation typed CRTs that the outer viewing screen side is formed with a flat shape and the inner phosphor-coated side with a curved shape.
In such a faceplate panel, as an inner curvature radius becomes smaller, the formation characteristics of the panel can be improved and the corresponding shadow mask can be formed with a stable structure capable of reducing a doming phenomenon. However, when the inner curvature radius of the panel falls below a minimum value, the peripheral portion of the panel is undesirably thick and this results in poor production efficiency as well as high production cost. Furthermore, the large thickness of the peripheral portion has a poor transmission rate and ultimately causes brightness failure.
In order to overcome such problems, various techniques are proposed for the one-sided flat panel CRT application. For example, these kinds of techniques are disclosed in Japanese Patent Laid Open Publication Nos. Hei 36710 and Hei 6-44926. However, they do not specify technical details for preserving the structural strength of the shadow mask which should be redesigned pursuant to the curvature radii varied at the inner side of the panel.
Furthermore, they do not calibrate the desired thickness ratios of a diagonal portion of the panel to the peripheral portion for minimizing distortion of the screen image. Therefore, when the CRT panel is manufactured on the basis of the above-identified techniques, the aforementioned problems remain.
In the usual sized flat-panel CRTs of 21-inch, 25-inch and 29-inch, it turns out that the thickness ratios of the peripheral portion of the panel to the center portion are 3.13, 2.91 and 2.72, respectively. These ratios are so high that they result in bad production efficiency as well as brightness failure.